Chemistry – Organic chemistry - Alkenes | e-Consult

In the reaction of ethene with bromine, the bromine molecule (Br₂) undergoes a reaction where the bromine atoms add across the double bond. The mechanism involves the bromine molecule being polarized by the pi electrons in the alkene, forming a bromonium ion intermediate. This intermediate is then attacked by a bromide ion (Br^-) from the opposite side of the molecule.

The key point is that the bromine atoms add to the *same* side of the double bond. This ensures that only one product is formed – 1,2-dibromoethane (BrCH₂CH₂Br). If the bromine were to add in a different way, leading to different arrangements of the bromine atoms, multiple products would be possible. The addition reaction proceeds in a concerted manner, meaning the bond breaking and bond forming occur simultaneously across the double bond, leading to a single product.